Carbapenemase Gene Transfer in CREC Hospitals

Study Background and Research Question

Carbapenem-resistant Enterobacter cloacae (CREC) is an important component of the broader carbapenem-resistant Enterobacteriaceae problem. Its clinical significance is not determined only by the presence of resistance in individual isolates. The location of carbapenemase-encoding genes (CEGs), their association with mobile genetic elements, and the ability of strains to disseminate between hospital settings all influence the persistence of resistance.

The reference study by Chen and colleagues addressed this problem using CREC isolates collected from eight teaching hospitals in Guangdong province, China, between December 2022 and June 2024. The authors focused on whether CEGs were located on plasmids, chromosomes, or both; whether the genes could be transferred by conjugation; and whether the isolates represented repeated clonal lineages or a more diverse population. These questions are particularly relevant to antimicrobial resistance research because plasmid-mediated resistance can spread between otherwise unrelated bacterial backgrounds.

The work was conducted against the clinical backdrop of the COVID-19 pandemic, when changes in antibiotic exposure, hospital operations, and patient complexity could alter selection pressure and transmission opportunities. Rather than treating resistance as a single phenotypic endpoint, the investigators examined the genetic architecture and epidemiological behavior of CREC. Their results are reported in Chen et al., BMC Microbiology (2025).

Key Innovation from the Reference Study

The central innovation is the integration of complementary evidence streams. PCR identified major carbapenemase genes, while variable-temperature SDS plasmid elimination helped distinguish plasmid-associated from chromosomal gene localization. Broth microdilution connected CEG status with antimicrobial susceptibility. Conjugation experiments tested whether resistance determinants could move into a recipient strain, and mobile-element analysis provided a genomic context for that movement. Finally, ERIC-PCR and NTSYS analysis assessed relationships among the clinical isolates.

This design is stronger than relying on PCR detection alone. A positive resistance gene result does not establish whether the determinant is mobile, whether it contributes to a broader multidrug-resistant phenotype, or whether identical strains are circulating across departments. By combining localization, transferability, phenotype, and strain relatedness, the study separates at least two mechanisms of persistence: horizontal dissemination of resistance genes and vertical or clonal dissemination of bacterial lineages.

The distinction matters for infection-control interpretation. A plasmid-associated gene that transfers efficiently may spread across multiple strain backgrounds, whereas a repeated ERIC-PCR pattern across hospitals may point to dissemination of a successful clone. The reference study therefore provides a practical framework for investigating the beta-lactam antibiotic mechanism of resistance at both the molecular and population levels.

Methods and Experimental Design Insights

The investigators analyzed 54 CREC isolates from the participating hospitals. CEG screening used PCR, and plasmid elimination with variable-temperature SDS treatment was used to infer whether detected genes were plasmid-borne, chromosomal, or present in both locations. This approach is useful for surveillance laboratories because it adds genetic-location information without treating all PCR-positive isolates as equivalent.

Antimicrobial susceptibility was measured by broth microdilution. The study compared CEG-positive and CEG-negative groups across several agents, including imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin. The method allowed the authors to test whether carbapenemase-gene carriage was associated with broader resistance rather than only with reduced carbapenem susceptibility.

Plasmid conjugation assays were then used to evaluate transfer potential. PCR analysis of transconjugants identified which genes had moved successfully. This is an important methodological distinction: conjugation demonstrates transfer under the assay conditions, but it does not by itself quantify transmission rates in patients or prove that transfer occurred between the sampled hospitals.

The study also characterized six mobile genetic elements and used ERIC-PCR to classify strain relatedness. NTSYS software grouped the isolates according to their fingerprint patterns. This combination of mobile-element analysis and genotyping helps distinguish gene mobility from clonal expansion, although ERIC-PCR provides a lower-resolution view than whole-genome sequencing.

Protocol Parameters

• Clinical isolate set: The literature-backed dataset comprised 54 CREC isolates collected across eight teaching hospitals during December 2022–June 2024; a comparable project should define the sampling period, participating facilities, specimen sources, and inclusion criteria before analysis.
• Gene detection: PCR was used to screen for carbapenemase-encoding genes. Primer selection, controls, and confirmation of amplicon identity should be specified in a reproducible laboratory workflow.
• Gene localization: Variable-temperature SDS plasmid elimination was used in the reference study to support plasmid-versus-chromosome assignment. Because this is an indirect localization strategy, confirmatory methods such as plasmid profiling or sequencing may be appropriate when available.
• Susceptibility testing: Broth microdilution compared resistance phenotypes between CEG-positive and CEG-negative isolates. Interpretive breakpoints should follow the current standard selected by the laboratory and should be reported with the testing platform.
• Transfer assay: Conjugation followed by PCR was used to assess movement of CEGs. Results should be described as experimental transferability, not as a direct estimate of in-host or hospital-wide transmission.
• Strain relatedness: ERIC-PCR and NTSYS clustering were used to examine relatedness. These tools are suitable for an initial epidemiological screen, while higher-resolution sequencing can strengthen conclusions about transmission chains.

Core Findings and Why They Matter

CEGs were detected in 46 of 54 isolates, corresponding to 85.19% of the collection, according to the reference study. The dominant determinant was bla_NDM-1. Eighteen isolates, or 33.33%, carried bla_NDM-1 on both chromosomes and plasmids, while 25 isolates, or 46.30%, carried it exclusively on plasmids. Two isolates carried only a plasmid-associated bla_IMP, and one carried both plasmid-associated bla_NDM-1 and bla_KPC-2.

The location data are important because plasmid-associated bla_NDM-1 creates a plausible route for horizontal gene transfer. In the conjugation experiments, CEG transfer was successful for 44 of 46 gene-positive isolates, a rate of 95.65%. Transfer was observed for 42 of 44 bla_NDM-1-positive transfer tests and both bla_IMP tests, whereas the single bla_KPC-2 test did not yield a transfer event. These findings support high transfer potential in the experimental system, particularly for bla_NDM-1, but should not be interpreted as a universal transfer rate for all clinical settings.

CEG-positive isolates also showed significantly higher resistance rates to imipenem, cefepime, gentamicin, ceftazidime/avibactam, ciprofloxacin, and levofloxacin than the CEG-negative group, with statistical significance reported by the authors. This pattern suggests that carbapenemase carriage occurred within multidrug-resistant backgrounds rather than as an isolated carbapenem phenotype. For bacterial infection model development, the result supports careful characterization of the starting strain: a model described only as carbapenem-resistant may conceal substantial variation in gene content and co-resistance.

Mobile-element analysis identified six element types, with ISEcp1 reported in 47 of the 54 isolates, or 87.04%. Twenty-two isolates, or 40.74%, carried four mobile-element types simultaneously. These observations provide a plausible mechanistic context for the observed mobility, although the presence of an insertion sequence does not alone establish the precise molecular event responsible for each transfer.

ERIC-PCR grouped the 54 isolates into 17 genotypes. Types E and G were each represented by 11 isolates, or 20.37%, and appeared in different departments across five hospitals. Two type E isolates had a Dice coefficient of 100%. Together, these data support a mixed transmission picture: mobile genes may move between strains, while related bacterial lineages may also persist or spread across institutional settings.

The epidemiological distribution was uneven. Men accounted for 35 isolates, older patients for 39, respiratory medicine for 11, and sputum for 18. These patterns describe where the sampled isolates were concentrated; they do not establish that sex, age, department, or specimen type independently caused CEG carriage. They are nevertheless useful for prioritizing surveillance and for identifying patient populations in which respiratory CREC screening may yield important information.

Comparison with Existing Internal Articles

The internal overview Carbapenemase Gene Transfer in CREC Hospitals emphasizes the same study’s integrated use of gene localization, susceptibility testing, conjugation, mobile-element analysis, and ERIC-PCR. Its value is as a concise map of the experimental logic; the present analysis adds caution about what each assay can and cannot establish, especially the difference between laboratory transferability and real-world transmission.

A second related resource, Transmission Dynamics of Carbapenemase Genes in CREC in Guangdong, places the findings in an antimicrobial-resistance and bacterial-surveillance context. Read together with the reference paper, it helps connect the high prevalence of plasmid-associated bla_NDM-1 with the need for molecular surveillance rather than phenotype-only reporting.

Limitations and Transferability

The study has several limits that affect generalization. The sample consisted of 54 isolates from eight teaching hospitals in one Chinese province and was collected during a defined period. The distribution may therefore reflect local prescribing, referral patterns, infection-control practices, and patient demographics rather than the global epidemiology of CREC.

Gene localization was inferred using plasmid elimination and PCR, and strain relatedness was assessed with ERIC-PCR. These methods are informative but generally provide less structural and phylogenetic resolution than long-read plasmid sequencing and whole-genome sequencing. Sequencing could clarify whether apparently related isolates share the same plasmid backbone, whether insertion sequences flank the resistance genes, and whether identical genes reside in different replicons.

The conjugation results demonstrate transfer under controlled experimental conditions. They do not measure transfer frequency in the human microbiome, environmental reservoirs, or hospital wastewater. Likewise, the cross-sectional design cannot reconstruct the direction or timing of transmission between hospitals. The epidemiological associations should therefore guide hypotheses and surveillance priorities rather than be interpreted as causal risk factors.

Finally, the study focused on selected carbapenemase genes and mobile elements. Other resistance mechanisms, porin changes, efflux activity, co-resistance genes, and antimicrobial exposure histories could further explain the observed phenotypes. These limitations do not weaken the central finding; instead, they define the next level of resolution needed for outbreak investigation and comparative genomic surveillance.

Research Support Resources

Researchers designing related antimicrobial resistance workflows can use Cefotaxime (SKU BA1012), a third-generation cephalosporin antibiotic, as a laboratory research reagent in appropriate susceptibility, selection, or bacterial infection model studies. The product is supplied as a solid and is listed for storage at −20 °C; freshly prepared solutions should be used promptly. Cefotaxime is intended for research use, not diagnostic or medical use, and its role should be defined by the assay design rather than inferred from the carbapenemase-transfer results in this paper.